1. Field of the Inventions
Embodiments of the present invention generally relate to mooring, riser and offloading systems for offshore hydrocarbon production developments. More specifically, embodiments of the present invention relate to the offloading of hydrocarbon fluids from a floating facility to an export tanker.
2. Description of Related Art
During the past thirty years, the search for oil and gas offshore has moved into progressively deeper waters. Wells are now commonly drilled at depths of several thousand feet below the surface of the ocean. In addition, wells are now being drilled in more remote offshore locations.
The drilling and maintenance of deep and remote offshore wells is expensive. In an effort to reduce drilling and maintenance expenses, remote offshore wells are oftentimes drilled in clusters. This allows a single floating rig or semi-submersible vessel to conduct drilling operations from essentially a single ocean location. Further, this facilitates the gathering of production fluids into a local production manifold after completion. Fluids from the clustered wells are commingled at the manifold, and delivered together through a single flowline. The manifold may be located subsea, or may be positioned on an offshore production platform.
From the manifold, produced fluids are delivered downstream by means of a subsea production flowline. The flowline carries the fluids to a processing facility, typically under pressure emanating from the originating subterranean reservoirs. The gathering facility collects and commingles fluids as produced from multiple wells.
In offshore fields where the metocean conditions are considered benign, the processing facility may be located offshore proximate to the subsea well-site. The coasts of West Africa, Indonesia, Malaysia and Brazil are examples of marine areas considered to have calm weather conditions. In such deepwater areas, a floating vessel may be the processing facility. Such vessels are referred to as “floating production vessels,” or FPV's. Such vessels are also sometimes referred to as “floating production, storage and offloading systems,” or “FPSO's.” For ease of reference, the term “FPV” will be used herein. FPV's may include equipment for treating fluids, such as by separating produced water from the produced hydrocarbons. The facility may further separate gas and liquid phase hydrocarbons before offloading. Produced hydrocarbons may be held at the FPV for future offloading and delivery to market.
It is desirable that an FPV maintain its geographic position offshore. The process of maintaining position offshore is called “stationkeeping.” To provide stationkeeping for the FPV, multiple sets of mooring lines can be used to secure the FPV to the ocean bottom. In areas of calm weather conditions, the mooring lines can be arranged in a “spread-mooring” pattern. For example, two sets of front lines may be provided, and two sets of rear lines may be provided. The lines may have a first end connected to the vessel, and a second point anchored at the ocean mudline. The various lines are typically 1 to 4 km in length, depending on water depth and other factors. Spread-moored systems keep the FPV headed in a single direction, oftentimes in the direction of the prevailing weather conditions. This eliminates the high cost of providing a turret mooring system that lets the FPV weathervane in response to wind, waves, and current.
In order to offload hydrocarbons from the FPV for delivery to market, a transport vessel or tanker is brought adjacent the FPV offshore. The bow of the tanker can be positioned behind the stern of the floating production vessel (tandem offloading). The two vessels may be tied together by a hawser line. A floating offloading hose is then connected from the FPV to the tanker in order to transfer fluids onto the tanker. The offloading conduit commonly ties into a midship manifold on the tanker. The close proximity of the tanker to the production vessel creates a hazard of contact. The potential result is loss of valuable hydrocarbons, damage to one or both vessels, and possibly even harm to the marine environment.
In an effort to mitigate this danger, an independent mooring system, which may include a surface offloading buoy, can be used for securing the position of the tanker relative to the FPV. One example is a catenary anchor leg mooring (CALM). The tanker positioning system can be a separate set of mooring lines, augmented by use of a tug boat or “tender vessel” connected to the tanker with tow lines. Tension is maintained in the towlines so as to maintain the tanker at an assured clear distance from the FPV. In some instances, dynamic positioning may also be employed on the tanker to maintain a safe distance.
The installation of deepwater mooring systems offshore for production vessels and offloading systems is expensive. Mooring systems have been offered as an alternative to a pure spread-mooring system. Examples of mooring systems are described in U.S. Pat. Nos. 5,639,187 and 6,571,723. Another example is the combined riser mooring system, or “CRM.” The CRM system integrates a riser mooring buoy into a spread-mooring system for the FPV. The CRM technology was published at the Deepwater Offshore Technology Conference in Stavanger, Norway in October 1999. Additional information relating to mooring, riser and/or offloading systems can be found in: U.S. Pat. No. 6,685,519; IE 990 872 A2; WO 03/013948; GB1 581 325 A; and Patent Abstracts of Japan, vol. 011, no. 157 (M-590), 21 May 1987 (May 21, 1987) & JP 61 287892 A (Mitsubishi Heavy Industries, Ltd.), 18 Dec. 1986 (Dec. 18, 1986).